The present invention relates to a heat-resistant alloy spring and a Ni-based alloy wire used therefor, which may be used suitably for apparatuses and devices used in high-temperature environments, e.g. pipe-switching valve inside a muffler in an exhaust system of an automobile engine, various heating furnaces and the like.
Heretofore, as metal alloy wires used for heat-resistant springs, stainless steel wires such as of SUS304, SUS631J1 and the like have been used on the ground of being comparatively inexpensive. The durable temperature thereof is however about 200 to 400 deg.C. at the highest.
Therefore, Ni-based alloy wires such as Inconel X750 and Inconel 718 have been widely used as heat-resistant alloy wires.
For example, “the anthology of preprints, pages 29-32, in the lecture presentation of the Japan society for spring Research in 1987 autumn” says that, as a result of a heat resisting property test at temperatures of from 450 to 500 deg.C., it was revealed that when compared with the spring of the conventional Inconel X750, the spring of Inconel 718 has about twice heat-resistance at an environmental temperature of 500 deg.C.
Thus, in the case of springs whose durable temperatures are about 500 deg.C. at the highest, there exist materials being relatively stable in the strength for the metal alloy wires used therefor. Accordingly, it is possible to make a choice from various materials in terms of material cost, workability, characteristics and the like.
In the case of various spring products used in for example engine exhaust systems such as of automobiles, aircrafts and the like, however, heat durability over the aforementioned temperature is required so as to be usable in high-temperature environments of over 600 deg.C. Further, there are necessitated such properties that the permanent set in thermal fatigue is less and the operating life is long even in the high-temperature environments.
For example, FIG. 3 shows a type of the muffler for an automobile engine. FIG. 3(A) shows the entire structure thereof in cross section. FIG. 3(B) is an enlarged view of a main part of an opening-and-closing valve thereof. The muffler 10 is provided in a metal casing 11 with separate chambers, in this example three chambers (A1 to A3). The chambers (A1 to A3) are communicated with each other through pipes (P1 to P4). The exhaust gas G fed thereinto from the pipe (inlet pipe) P1 flows in the respective chambers as indicated by arrows in the figure, and finally discharged from the pipe (exhaust pipe) P4. As shown in FIG. 3(B), the above-mentioned heat-resistant spring S is used as a spring for pressing a lid 13 of the opening-and-closing valve 12 disposed (within the chamber A1) at an end of the pipe P2. In this example, through an open end B2 and side holes B1 of the inlet pipe P1, the exhaust gas G fills the chambers A2 and A3. up to a certain feed rate, the filling gas G flows out into the chamber A1 and is discharged from the exhaust pipe P4. But, when the feed rate becomes over a preset value, the opening-and-closing valve 12 opens, and the pipe P2 becomes a bypass channel. Spring property of the spring S is adjusted according to the opening-and-closing valve 12.
The exhaust gas G fed into the muffler 10 is high-temperature gas combusted in the engine. In consequence, the above-mentioned spring s is required to have a heat resistance enabling to withstand such high temperatures and the spring performance enabling to maintain the preset value of the feed rate of the exhaust gas G. Further, the thermal fatigue rate is required to be small sodas to restrain the occurrence of permanent set in thermal fatigue by withstanding the heat. And also, a longer operating life is necessitated.
The “thermal fatigue rate” means the value obtained by: deforming a spring by applying a prescribed stress; obtaining the load P1 of the spring at the time; exposing the spring, with keeping the deformed state (h), to a high-temperature environment for a predetermined time; releasing the spring thereafter; loading the spring and obtaining the load P2 by which the spring becomes the same deformed state (h) as before; dividing the decrease of the load (P1−P2) by the original load P1; and being expressed in percentage. This is also called load loss WS (%) and can be expressed in the following expression:WS={(P1−P2)/P1}×100.
In the case of a helical compression spring for example,    P1 is the load (N) of the spring at a height (h) corresponding to a striction strain of 600 MPa before exposed to high temperature,    P2 is the load (N) at the spring height (h) after exposed to the high temperature.
Meanwhile, the smaller value Ws is better for the heat-resistant spring.
In connection with the heat-resistant springs for such a use, a heat-resistant Ni alloy wire comprising 0.01 to 0.40% C, 5.0 to 25.0% Cr and 0.2 to 8.0% Al; at least one constituent selected from among 1.0 to 18.0% Mo, 0.5 to 15.0% W, 0.5 to 5.0% Nb, 1.0 to 10.0% Ta, 0.1 to 5.0% Ti and 0.001 to 0.05% B; the balance being Ni which includes at least one constituent selected from among 3.0 to 20.0% Fe and 1.0 to 30.0% Co; and incidental impurities, have been proposed in Japanese Patent No. 3371423 (hereinafter referred to as Document 1). This heat-resistant Ni alloy wire is descried as being usable under such a condition that the durable temperature is 700 deg.C. or less because the tensile strength and crystal grains are controlled.
Further, Japanese Patent Application publication No. 2000-345268 (hereinafter referred to as Document 2) proposed that, in a Ni-based alloy wire having a composition similar to that of Document 1 but Zr is further added, the grain size number and surface roughness are specified. This is described as being possible to set the residual shearing strain below 0.3% at an environmental temperature of 700 deg.C.
Furthermore, a spring alloy has been proposed in Japanese Patent No. 3492531 (hereinafter referred to as Document 3) wherein, in a heat-resistant stainless steel, a weight ratio “{eta phase [Ni3 Ti: hcp structure]/gamma prime phase [Ni3 (Al, Ti, Nb)]}×100” of eta phase [Ni3 Ti: hcp structure] precipitated at grain boundaries and gamma prime phase [Ni3 (Al, Ti, Nb)] precipitated in matrix of gamma phase crystal grains is set in a range of from 0.01 to 10.00%. This gamma prime phase [Ni3(Al, Ti, Nb)] is 1 to 20 nanometer spherical grains. The above-mentioned gamma phase means austenite.
Various recent devices are required to be downsized and to be high-performance. For example, in the case of the above-mentioned high-temperature spring used with a car engine or the exhaust system, it is required to be useable at environmental temperatures higher than ever (for example 700 to 800 deg.C.), without substantial deterioration of the spring properties and the mechanical strength. In such environmental temperatures, however, it is difficult to employ the above-mentioned hitherto-proposed heat resistant materials.
That is to say, the heat-resistant alloy wire in Document 1 is of a Ni-based alloy having been known as a high-temperature material such as Inconel X-750 and Inconel 718. The tensile strength, crystal grains and aspect ratio are defined within specific ranges, aiming at improvements in the properties. Moreover, an example in Document 1 is described as having a residual shearing strain of 0.3% at a compressive strain of 600 MPa and a temperature of 650 deg.C.×24 hours.
However, when that in Document 1 is used at temperatures higher than their operating temperature 650 deg.C., for example, that is used in a high-temperature environment of 700 deg.C., there is possibly that the residual shearing strain becomes larger and the operating life becomes shorten. Namely, even though it is explained with the example that the residual shearing strain at the environmental temperature of 650 deg.C. is 0.2 to 0.37%, it can not be said that Document 1 refers to the property under a temperature environment over 650 deg.C.
Further, in Document 1, the crystal grain diameter and the aspect ratio are defined, but, as to the criteria, there is no concrete explanation. Furthermore, regarding the composition, a broad range is given to the content of each element, therefore, it is presumable that the crystal structure and the state of the material become nonuniform. Therefore, the definition of the ranges for the crystal grain diameter and the aspect ratio do not make much sense.
Document 2 states that the heat resistance is relatively stable under an environmental temperature of 650 deg.C. similarly to that in Document 1. But, the shearing strain at 700 deg.C. is remarkably increased. Accordingly, it is presumed that a critical region of the characteristic change lies between these temperatures. But, optimum conditions in this region between these temperatures cannot easily be determined from the description of Document 2.
The above-mentioned Document 3 shows a wire of a stainless steel whose Ni content is 10 to 50 wt %—in the examples, 25 wt % and 35 wt %. In this case, it is presumable that a precipitation quantity of gamma prime phase [Ni3(Al, Ti, Nb)] whose major component is Ni, will be relatively few. Even if the grain diameter becomes small, it is difficult to obtain a sufficient heat-resisting effect.